desu is like a statement and deshou is asking for agreement.

e.g.

anata wa otoko desu.
you're a guy.

(anata wa) otoko deshou?
you're a guy, aren't you?

It's kinda optional, you can pronounce it desu or des. Generally 'u's and 'i's are schwas and you don't have to pronounce them out.

mitai can also mean something like want to try.

e.g.

sore wo yatte mitai
I want to see if I can do that.

Try learning the grammar first. There are a lot of resources out there. Once you get used to the structure of the language, you'll learn very quickly as all you need is to expand your vocab and learn cultural references.

i think "ochinko" = penis...

btw, the suggestions for sumimasen / gomen, etc. on top are a bit odd imo.

Want to tell/convey

Well, this is what I think. Sumimasen is considered more polite and more of an 'excuse me', type of word, but can also be used to as an apology word. Gomen is just to apologise. Sumimasen is more appropriate when apologising to strangers.

As for shitsurei shimasu and shimashita, the earlier is present/future tense and the latter is past tense. While or before you'd do something, you'd use shimasu. And for a mistake that you'd done last time, you'd use shimashita, otherwise they might think you're excusing yourself to go home or something.

@eisenmeteor

その私はそれのために行くまたは私はそれを忘れている to me would mean:

Talking about that me, talking about that purpose of the again going, talking about me, i forgot that.

"Its either i go for it or i forget it"
maybe you can try, "頑張らないとダメだ" for something simpler